789B Off-Highway Truck Air System and Brakes Caterpillar

Illustration 1	g00394517
Location of parking and secondary brake valve

Illustration 2	g00277905
Parking and secondary brake valve in the ON position (1) Passage from parking brake release pump section (2) Relief valve (4) Piston (5) Air inlet passage (6) Stem (7) Drain passage to the front brake cooling circuit (8) Drain passage to hydraulic tank (9) Check valve (10) Spring (11) Outlet passage for the oil that releases parking brakes

The parking and secondary brake valve is located on the left side of the truck frame near the fuel tank. The parking and secondary brake valve controls the flow of oil to the parking brakes.

The spring applied parking brakes are released by hydraulic pressure. The engine must be in operation in order to have oil pressure in passage (1) from the parking brake release pump section.

The parking brake air control valve and the secondary brake air control valve are located in the cab. When either of these controls are in the ON position, air flow to air inlet passage (5) will be stopped. When there is no air flow to air inlet passage (5) , spring (10) will keep stem (6) in place. This blocks the flow of oil from passage (1) to passage (11) . When no oil flows to the parking brakes, a spring will apply the parking brakes.

Illustration 3	g00277906
Parking and secondary brake valve in the OFF position (1) Passage from parking brake release pump section (2) Relief valve (4) Piston (5) Air inlet passage (6) Stem (7) Drain passage to the front brake cooling circuit (8) Drain passage to hydraulic tank (9) Check valve (10) Spring (11) Outlet passage for the oil that releases parking brakes

The parking brake air control valve and the secondary brake air control valve are located in the cab. When both of these controls are in the OFF position, air flow will be sent to air inlet passage (5) . Air pressure forces piston (4) and stem (6) downward. When stem (6) moves downward, pressure oil from passage (1) enters passage (11) . When this pressure oil flows to the brake housing, the pressure oil compresses a spring. This releases the parking brakes. Relief valve (2) limits system pressure.